This invention is directed to a process for preparing N-methyliminodiacetic acid, and in particular, a process for recovering the same from solutions of its disodium salt.
In the prior art, amino acids such as glycine and B-alanine have been prepared by hydrolyzing the corresponding nitrile with an aqueous alkaline earth metal hydroxide to form an alkaline earth metal salt of the amino acid, and treating the alkaline earth metal salt with carbon dioxide to form the free amino acid, and recovering the amino acid.
Another method of producing such amino acids includes treatment with sulfuric acid to convert the intermediate sodium salt (such as sodium glycinate in the case of glycine) to the free amino acid. However, such a process results in the formation of sodium sulfate, which is difficult to separate from the free amino acid.
U.S. Pat. Nos. 3,904,585 and 3,947,496 to Thunberg et al., the disclosures of which are incorporated by reference, disclose a process for separating glycine or B-alanine from such sodium sulfate solutions by fractional crystallization. Specifically, in the starting aqueous solution of sodium sulfate and the amino acid, which has a temperature above 33.degree. C., a pH of 4.5-8.5, a mole ratio of amino acid to sodium sulfate of 1-5:1, and containing at least 5% of the amino acid, water is evaporated at a temperature of from 60.degree. C. or 70.degree. C. to about the normal boiling point in order to form a first slurry which is a mixture of precipitated sodium sulfate and a first mother liquor. The temperature is such that the precipitation of the amino acid is prevented. Upon separation of the precipitated sodium sulfate, the mother liquor is cooled to a temperature effective for precipitating the amino acid. The amino acid is separated and recovered, and the process is repeated.
U.S. Pat. No. 3,808,269 to Bragdon et al. discloses a similar process, except that iminodiacetic acid is separated from sodium sulfate solutions.
Methyliminodiacetic acid is of interest for photographic applications. In Chemical Abstracts 111(8):67804u, for example, it is disclosed that a bleaching solution for bleaching an exposed photographic material contains Fe(III) complex salts of organic acids including methyliminodiacetic acid. Similarly, in Chemical Abstracts 93(6):588196e, a bleaching solution comprised of FeCl.sub.3, KBr and methyliminodiacetic acid, adjusted to a pH of 6.0 with NH.sub.3, is disclosed. In Chemical Abstracts 77(4):24608v, the biodegradation of N-methyliminodiacetic acid in river water is disclosed.
Although other, perhaps more effective, chelating agents exist, N-methyliminodiacetic acid is biodegradable, thus making its use highly desirable in terms of environmental concerns However, where N-methyliminodiacetic acid is formed from its disodium salt by acidification with sulfuric acid, separation of the acid from the resulting sodium sulfate solution is problematic. None of the foregoing patents suggests the application of fractional crystallization for the separation of N-methyliminodiacetic acid from such solutions.